Up-conversion or anti-Stokes processes occur in materials capable of absorbing photons at energies lower than the subsequently emitted photons. These materials can convert near-IR radiation to light in the visible part of the electromagnetic spectrum. In up-conversion processes either single- or multi-photon excitations take place. A well-known type of an up-conversion process is the excitation of anti-Stokes emission bands in Raman spectroscopy, where a lower energy photon is converted to a higher energy one and the additional energy required is provided by lattice when one or more phonons are annihilated. Anti-Stokes bands occur at specific energies determined by the phonon spectrum of the host lattice. A vibronic ground state absorbs an excitation photon and the emission occurs from a so-called virtual excited state into the ground state. Typically the emission occurs at energies which are 100-1000 cm−1 higher than the energy of the excitation photon. This can be used to infer the phonon spectrum of the material since the energies of phonons are quite often well-defined. This is a very inefficient process: in Si the anti-Stokes Raman emission has an efficiency of 10−13 cm2W−1.
In a two-photon up-conversion process a virtual intermediate state exists, while both ground and excited states are real. Spontaneous emissions of photons during the decay of excited atoms are prohibited by ordinary quantum mechanics and require the quantization of the electromagnetic field. The two-photon up-conversion process requires that the sum of energies of the two exciting photons be larger than the band gap energy and the simultaneous absorption of two photons is an inherently less efficient process than single photon absorption. The efficiency of two-photon absorption of the red-light from a pulsed ruby laser in CaF2:Eu2+ resulting in blue photons is ˜10−12 cm2W−1. Another two photon process is second harmonic generation: exciting KH2PO4 or KNbO3 with 1064 nm laser light from a Nd3+ YAG an emission in the green at 532 nm is observed. In this case the two photons need to be coherent and both the intermediate and the excited state are virtual states with a zero life time.
Another process relies on the sequential absorption of two photons by two different activator ions and their subsequent decay into their ground states from a virtual excited state while emitting photons with energies equal to the sum of the energies of the individual ions. This cooperative luminescence has been observed in Yb 3+: YbPO4 where it occurs with an efficiency of ˜10−8 cm2W−1 when two exited Yb3+ activators generate a single photon emission in the green part of the spectrum. In a similar process two excited photons are sequentially absorbed and raised into their excited states where they are then transferred to another activator bringing this ion to an excited state with energies equal to the sum of the two excited activators. The final state is a real and therefore this cooperative sensitization is more efficient than cooperative luminescence. In Yb3+, Tb3+: YF3 the sensitization of Tb3+ from Yb3+ activators has an efficiency of 10−6 cm2W−1.
Three photon up conversion processes are also known: in SrF2: Er3+ up to three 1 μm photons can be absorbed sequentially by Er3+ allowing emission in the blue, green or red regions of the electromagnetic spectrum. In the process a real intermediate state is needed since only at a finite life time of this state can the initial excitation be further promoted by a second excited photon.
Other systems where three-photon absorption and up-conversion processes have been established are Er3+:YF3 and Tm3+, Yb3+:NaYF4. In the latter system 4-photon processes are also possible.
Frequency up-conversion luminescence has great potential applications in diode-pumped all-solid-state visible laser, highly efficient next generation lighting, near-IR photon detectors, high power fiber up-conversion lasers, nm-sized biological labels and new emissive displays. Up-to-date rare earths have mainly been doped into oxyfluoride glasses since oxyfluorides combine the favorable low phonon energy and high up-conversion efficiencies with the high mechanical and chemical stabilities of oxides.
A need exists for improved materials, devices, and methods that utilize up-conversion to absorb photons at energies lower than their subsequently emitted photons.